1. Technical Field of the Invention
The present invention generally relates to telecommunications network equipment. More particularly, and not by way of any limitation, the present invention is directed to protective circuitry with respect to a Subscriber Line Interface Circuit (SLIC).
2. Description of Related Art
Subscriber Line Interface Circuits (SLICs)are employed to provide an interface between a low voltage signal path in a telephone central office (CO) and a high voltage telephone subscriber line. The SLIC provides functions such as off-hook detection, ringing, signaling generation and battery feed to the subscriber line. The subscriber line consists of a telephone transmission line, including two conductors referred to as A and B or TIP and RING, and the subscriber telephone equipment coupled therebetween. The subscriber line and the subscriber telephone equipment are also referred to as a subscriber loop.
The SLIC provides power from the telephone CO, or a remote office, to the subscriber line in response to a received battery voltage. Whereas single supply SLICs have been used traditionally, SLICs with multiple power supplies (e.g., dual supplies) are becoming commonplace in today's access networks. Typically, a high positive voltage source is used as one power supply and a negative voltage source provided by a battery is used as the other power supply. Using such a power supply scheme, a differential voltage signal of a substantially high magnitude necessary for balanced ringing may be developed.
One of the potential hazards of implementing a high positive voltage source is that when the SLIC is electrically damaged for any reason, one of the TIP/RING terminals may be shorted to the voltage source. As a consequence, the subscriber loop conductor(s) may carry a dangerous voltage level to subscriber premises even in a non-ringing condition, thereby violating applicable safety standards (e.g., the UL standards). Another concern with respect to SLICs used in conjunction with sensitive Digital Subscriber Line (DSL) circuitry is the need to comply with the transient tests specified in GR-1089 while still protecting the DSL circuitry against overvoltage conditions. Solutions that address these and other shortcomings are either nonexistent or, where available, generally deficient in terms of high cost and the requirement of additional components and board space.